Hydrogen gas (hereinafter referred to simply as hydrogen) has received attention in recent years as a clean energy source that emits no green house effect gas such as CO2 when burned. Since little hydrogen exists in the air (1 ppm or less), normally it is manufactured by steam-reforming hydrocarbons.
In this hydrogen manufacturing process, carbon monoxide (CO) and/or carbon dioxide (CO2) are produced together with hydrogen (H2) while a hydrocarbon (such as methane: CH4) as an unreacted raw material gas and steam (H2O) remain. Subsequently, hydrogen is separated from a mixture of these gases and purified.
Representative hydrogen separation/purification methods include pressure swing adsorption (PSA) and membrane separation. While PSA allows production of high purity hydrogen with the use of a plurality of adsorption towers, it has a disadvantage of tending to require a large and complicated system. On the other hand, membrane separation, though having an advantage of requiring a small and simplified system, has a disadvantage of high material costs because it has conventionally used palladium-based alloy membranes (e.g., Pd—Ag alloy membranes) as a separation membrane.
Therefore, various studies have been conducted on inexpensive metallic membranes as potential substitutes for palladium-based alloy membranes and hydrogen separation devices using such inexpensive metallic membranes. As a result, it has been reported that vanadium (V), niobium (Nb), and tantalum (Ta) each have high hydrogen permeability as a metal simple substance, and hydrogen separation alloy membranes of multi-phase alloys of those metals and other metals (e.g., titanium (Ti), nickel (Ni), cobalt (Co), zirconium (Zr), and hafnium (Hf)) exhibit high hydrogen permeability.
In particular, Nb—Ni—Ti-based alloy membranes are attracting attention for their high general potentials in terms of hydrogen permeability and hydrogen embrittlement resistance (see, for example, Non-Patent Literature 1).
However, there is a problem that in the case where any hydrogen separation membrane other than palladium-based alloy membranes is used in a hydrogen separation device, when the device is stopped (or when the temperature of the device is lowered) with hydrogen present around the hydrogen separation alloy membrane, the hydrogen separation alloy membrane degrades (so-called hydrogen embrittlement). In order to solve such a problem, various hydrogen separation devices and control methods have been studied and suggested.
For example, Patent Literature 1 (JP 2001-118594 A) discloses a fuel cell system for generating power by the reaction between hydrogen and oxygen. When the fuel cell system is stopped, the supply of fuel to a reforming apparatus for producing a reformed gas with fuel and air is shut off. At the same time, air is introduced into the side of the reforming apparatus of a hydrogen separation alloy membrane connected downstream of the reforming apparatus for separating only hydrogen from the reformed gas, and a valve of a hydrogen supply line connected to the permeate side of the hydrogen separation alloy membrane is closed. This removes hydrogen on the side of the reforming apparatus and on the permeate side of the hydrogen separation alloy membrane.
According to Patent Literature 1, when the fuel cell system is stopped, hydrogen, which causes degradation of a hydrogen separation alloy membrane, in the hydrogen separation alloy membrane can be positively and easily removed, and a difference of pressure between both electrodes of the fuel cell, which causes degradation of fuel cell components, can be suppressed without using an inert gas (for example, nitrogen gas).
Patent Literature 2 (JP 2003-112905 A) discloses a fuel reforming system provided with a water vaporizer for producing steam and a reformer for producing a hydrogen-rich reformed gas by the reformation reaction between fuel and water. The reformer is a membrane reactor composed of a reforming layer and a pure hydrogen layer that are adjacent with at least a hydrogen separation alloy membrane therebetween. When the system is stopped, only steam produced by the water vaporizer is supplied to the reforming layer and the pure hydrogen layer so as to maintain a predetermined temperature at which hydrogen embrittlement in the hydrogen separation alloy membrane can be avoided. After steam is supplied in the amount necessary to purge the residual gas in the reforming layer and the pure hydrogen layer, air is supplied to the reformer.
According to Patent Literature 2, before conducting the purge using air when the system is stopped, conducting the purge using steam eliminates the need for conducting a purge using an inert gas. As a result, any storage means such as a cylinder is not necessary for the fuel reforming system, which makes the fuel reforming system more space efficient. Moreover, when any residual gas, such as hydrogen, is present in the fuel reforming system, the hydrogen separation alloy membrane is maintained at temperatures at which hydrogen embrittlement does not occur. Consequently, hydrogen embrittlement in the hydrogen separation alloy membrane can be prevented.
Patent Literature 3 (JP 2003-334417 A) discloses a protection method for a device using a hydrogen permeable membrane. The device has a hydrogen-permeable membrane composed of a hydrogen-permeable metal or alloy. In the device, pipes connected to the space on the hydrogen-supply side and the space on the hydrogen-permeate side of the hydrogen permeable membrane are each provided with one or more temperature sensitive valves using heat as a power source, which are operated by the heat of the device itself. When the device is started, these valves detect the temperature rise of the device and automatically enter into a steady operation state. When the device is stopped, the valves detect the temperature drop of the device and automatically operate so as to remove hydrogen in the spaces connected to the hydrogen permeable membrane.
According to Patent Literature 3, since there is no need to expose the hydrogen permeable membrane to hydrogen at temperatures below the service temperature limit, fractures in the membrane can be prevented. Moreover, by using temperature sensitive valves using heat as a power source, there can be provided a system that does not consume any specific electrical energy for valve operations necessary for starting/stopping a hydrogen permeable membrane and does not require any electronic control.